Is there a reason why you need to replace the bearings? Because that job requires complete disassembly of the lower unit and requires special tools.
Water intrusion may not necessarily have harmed the bearings unless it's been sitting with nothing but salt water in the gearcase for a long time. If you instead have an emulsion of water and oil you may or may not have damage.
The best thing to do there is to pull the L/U, remove the water pump assy including the pump base (which holds the driveshaft seals) and then you'll be able to evaluate the condition of the driveshaft ball bearing. If the bearing looks good and operates smoothly, the propshaft turns smoothly and the L/U shifts properly, you may need to do nothing but a re-seal.
The most likely places to leak are the driveshaft seals and the shift shaft bushing seal. You can do a pressure test on the gearcase, if you have the right equipment. 12psi max pressure.
I would pick up the complete pump kit, which includes upper and lower pump housings, impeller, and all the sundries.
The lower pump housing will come with oil seals already installed, saving you some time & effort.
Also pick up a new shift shaft bushing, it'll be easier for you and comes with new seal already installed, and a new O-ring. You'll need the special bushing removal tool or make your own by grinding "tabs" in an appropriately-sized deep socket.
NOTE: if you do pull the bushing, DO NOT allow the shift shaft to move upwards or pull the shift shaft out, the shift clutch is spring-loaded and puts pressure on the shift cam, when you pull the shift shaft out of the cam the spring pressure will push the shift cam out-of-alignment and you'll have to go thru some machinations (i.e. stand on your head and hold your mouth right, etc
) to get it back in place. I can help with that if it happens anyway!
Some P/N's for the bushing removal tool are: Merc 31107T, Sierra 18-9817, GLM 901040.
Your 1977 50hp Merc should have the "hockey-stick" style of blue & red decals. 1977 model year for 50hp started with serial #4576237. Check your serial #, located on the plate between the transom clamps, and plug it into the Merc parts database here:
http://www.marineengine.com/parts/mercury-outboard/index.php
This should lead you directly to the parts diagrams for your motor.
HTH & let us know what you find when you tear into it......ed
